A large portion of the data storage in today's computers uses magnetic media such as magnetic disks. Data is presented to a computer by huge numbers of bits (ones and zeroes) and stored on disks where each bit is represented by a transition, which causes an applied magnetic field. In order to read or write the value of any given bit, a read/write sensor is used, which includes one portion for changing or writing to the disk and another portion for detecting or reading changes in the applied magnetic field.
In the read portion, a sensor that changes electrical resistance in response to a magnetic field, called a magnetoresistive (MR) sensor, is employed. In the past, sensors utilized the anisotropic magnetoresistive (AMR) effect where a read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. The sensor reads data from magnetic transitions recorded in the media. The magnetic field, resulting from a transition, causes a change in the direction of the magnetization in the read element. The new magnetization direction changes the resistance of the read element with a corresponding change in the sense current or voltage.
More sensitive sensors used a larger form of magnetoresistance called the giant magnetoresistance (GMR) effect. The GMR effect occurs in multilayer thin films of alternating ferromagnetic and nonferromagnetic metals. The resistance of a GMR film changes according to the cosine of the angle between the magnetization of the ferromagnetic (FM) layers.
The most commonly used sensors now are a subset of the GMR devices called a “spin valve” in which two ferromagnetic layers, a “free” layer and a “pinned” layer, are used. When the magnetization in the two layers are aligned, the resistance is at a minimum. When the magnetization is anti-aligned, the resistance is at a maximum. The resistance varies as the cosine of the angle between the magnetizations and is independent of the direction of current flow. The magnetization of the pinned layer is held in place by depositing it next to a layer of antiferromagnetic (AFM) material with a resulting exchange coupling of the two layers. The free layer magnetization is free to rotate in response to the field from the disk. In this way, the magnetization swings between being parallel (low resistance state) to anti-parallel (high resistance state) as the head flies over recorded magnetic transitions on the disk. The resulting change in electrical resistance arising from the GMR effect is sensed and the magnetic information on the disk is transformed into electrical signals.
Newer even more sensitive sensors are required which are more sensitive to smaller recorded transitions on higher density media. However, previous sensors have reached their limits so new technologies are being investigated. One new technology that has previously unsuccessfully been investigated is called “spin-dependent tunneling (SDT)” sensor. An SDT sensor differs from the older spin valves by having an insulator between the two magnetic layers. The current passes through perpendicular to the trilayers as compared to parallel in the case of a spin valve sensor. This perpendicular flow makes it difficult to provide a hard bias to stabilize the domains in the sensor; however, it is believed that an SDT sensor would be more sensitive than the current state of the art.
The problem with SDT sensors is that the hard bias requires a hard or permanent magnet, to pin edge of the free layer and the hard magnet interferes with the tunneling current, which is perpendicular to the layers. This hard bias is referred to as “longitudinal hard bias” because it is directionally parallel to the longitudinal direction of the disk media surface.
Problem with obtaining a satisfactory longitudinal hard bias is one of the biggest problems in the industry and is one of the reasons that it has not been possible to produce a dependable recording head using this spin-dependent tunneling technology.
Some attempts have been made to make a spin-dependent tunneling sensor using a hard magnet at the sides of the free layer, which is outside of the tunneling magnetoresistance junction. Unfortunately, previous systems required two free layers in this configuration, which added to complexity and cost.
Another structure involved an insulator between the free layer and the hard biasing magnet, but this resulted in the requirement for a thick insulator because the hard biasing strength becomes smaller in proportion to the thickness so that stabilization is inefficient. Conversely, attempts to make the insulator thinner resulted in short-circuiting of the sensor.
Solutions to problems of this sort have been long sought, but have long eluded those skilled in the art.